Substrate for liquid filter

ABSTRACT

A substrate for a liquid filter, which includes a polyolefin microporous membrane, the polyolefin microporous membrane having a water permeation efficiency of 1.21 to 2.90 ml/min·cm 2 , the polyolefin microporous membrane having a bubble point of 0.40 MPa to 0.65 MPa, the polyolefin microporous membrane having a compressibility of less than 15%.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Stage of International Application No. PCT/JP2014/062126 filed May 2, 2014 (claiming priority based on Japanese Patent Application Nos. 2013-097632 filed May 7, 2013, and 2013-097635 filed May 7, 2013), the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a substrate for a liquid filter.

BACKGROUND ART

In recent years, electronic devices are becoming increasingly smaller with higher performance. In particular, digital devices and portable terminals, as typified by personal computers and smartphones, have been dramatically developed. Among various techniques leading and supporting such development, as is well known, technological innovation in the semiconductor industry has played a major role. In the recent semiconductor industry, the development race is going on in the field where the wiring pattern dimension is below 20 nm, and the manufacturers have been accelerating the construction of cutting-edge production lines.

A lithography step is a step of forming a pattern in the production of semiconductor parts. With the recent development of finer patterns, an extremely high level of technology has been required not only in terms of the nature of the liquid chemical used in a lithography step itself but also for the handling of the liquid chemical prior to application onto a wafer.

A liquid chemical prepared at high level is filtered through a dense filter immediately before application onto a wafer to remove particles that would greatly affect the pattern formation or yield. In the cutting-edge formation of patterns of less than 20 nm, the capability of collecting particles of less than about 10 nm is required, and filter manufacturers are vigorously advancing the development.

In addition, also in the field where the wiring pattern dimension is over 20 nm (e.g., 30 nm to 100 nm), which has been the cutting-edge field in the past, as a requirement for a liquid chemical necessary to form a wiring pattern, the elimination of conventional microcontaminants from outside is demanded. Further, there have also been demands for the elimination of gel-like matter resulting from gelation due to the high reactivity of the liquid chemical itself and also for the maintenance of the purity of the highly purified liquid chemical, that is, the protection of the liquid chemical from contamination. In the production of such semiconductors of various wiring pattern dimensions, the removal of particles having a size of about 50 nm to 100 nm is necessary.

Generally, a liquid filter includes, as a substrate, a porous membrane made of a resin such as polyethylene, polytetrafluoroethylene, nylon, or polypropylene, and is processed into the form of a cartridge and used. Substrates are used according to the intended application in terms of compatibility with the liquid chemical, collection efficiency, processing capacity, life, and the like. Recently, the reduction of dissolved matter derived from the substrate has been particularly emphasized, and polyethylene microporous membranes are commonly used as substrates.

As typical methods for producing a polyethylene microporous membrane, a phase separation method and a drawing method can be mentioned. A phase separation method is a technique that forms pores utilizing the phase separation phenomenon of a polymer solution. Examples thereof are a heat-induced phase separation method in which phase separation is induced by heat as described in PTL 1, a non-solvent-induced phase separation method utilizing the solubility characteristics of a polymer to the solvent, etc. In addition, it is also possible to combine the two techniques, heat-induced phase separation and non-solvent-inducted phase separation, or to further adjust the shape or size of the pore structure by drawing to increase the variation. A drawing method is, for example, as described in PTLs 2 to 5, a method in which a polyethylene raw sheet formed in a sheet shape is drawn to extend the amorphous part in the crystal structure under adjusted drawing conditions including the speed, ratio, temperature, etc., thereby forming micropores between lamella layers while forming microfibrils.

CITATION LIST Patent Literature

PTL 1: JP-A-2-251545

PTL 2: JP-A-2010-053245

PTL 3: JP-A-2010-202828

PTL 4: JP-A-7-246322

PTL 5: JP-A-10-263374

SUMMARY OF INVENTION Technical Problem

However, when particles of about 50 nm to 100 nm are to be efficiently collected, conversely, the liquid permeability tends to decrease. That is, there is a trade-off relationship between collection efficiency and liquid permeability.

In addition, in the long-term use of a liquid filter, due to repeated application of pressure to the polyolefin microporous membrane, the porous structure may change, resulting in a gradual decrease in the liquid permeability. In order to solve such a problem in long-term stable use, it is possible to provide a polyolefin microporous membrane with a rigid structure, for example. However, this also affects the collection efficiency or liquid permeability. In addition, in the case where the liquid filter is used over a long term, or in a hot-pressing (bonding) step during processing into a filter, for example, there may be problems in that the polyolefin microporous membrane is needlessly crushed, whereby the original porous structure cannot be maintained, resulting in unstable water permeation efficiency, or that the filtration pressure abnormally increases due to pore blockage, for example. Accordingly, there has been a demand for a polyolefin microporous membrane capable of maintaining an excellent porous structure in the long-term use or processing of the liquid filter.

Further, in the related art such as PTLs 1 to 5, no proposal has been made to achieve excellent collection efficiency for particles of about 50 nm to 100 nm together with excellent liquid permeability, and further also achieve stable liquid permeability for long-term use.

Thus, in order to solve the problems mentioned above, an object of the invention is to provide a substrate for a liquid filter, which has excellent collection efficiency for particles of about 50 nm to 100 nm together with excellent liquid permeability, and also has stable liquid permeability for long-term use.

Solution to Problem

In order to solve the problems mentioned above, the invention is configured as follows.

1. A substrate for a liquid filter, including a polyolefin microporous membrane,

the polyolefin microporous membrane having a water permeation efficiency of 1.21 to 2.90 ml/min·cm²,

the polyolefin microporous membrane having a bubble point of 0.40 MPa to 0.65 MPa,

the polyolefin microporous membrane having a compressibility of less than 15%.

2. The substrate for a liquid filter according to 1 above, wherein the polyolefin microporous membrane has a heat shrinkage of 20% or more in the width direction after a heat treatment at 130° C. for 1 hour.

3. The substrate for a liquid filter according to 1 above or 2, wherein the polyolefin microporous membrane has a thickness of 7 to 16 μm.

4. The substrate for a liquid filter according to any one of 1 to 3 above, wherein the polyolefin microporous membrane has a pore-blocking temperature of more than 140° C.

5. The substrate for a liquid filter according to any one of 1 to 4 above, wherein the polyolefin microporous membrane has a porosity of 50 to 60%.

Advantageous Effects of Invention

The invention enables the provision of a substrate for a liquid filter, which has excellent collection efficiency for particles of about 50 nm to 100 nm together with excellent liquid permeability, and also has stable liquid permeability for long-term use.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described in sequence. However, these descriptions and examples are illustrative of the invention, and do not limit the scope of the invention. Incidentally, throughout this Description, a numerical range using “to” includes its upper limit and lower limit. In addition, with reference to a polyolefin microporous membrane, the term “length direction” means the direction of the length of the polyolefin microporous membrane that is produced in an elongated shape, while the term “width direction” means the direction that is perpendicular to the length direction of the polyolefin microporous membrane. Hereinafter, “width direction” is sometimes also referred to as “TD”, while “length direction” as “MD.”

[Substrate for Liquid Filter]

The substrate for a liquid filter of the invention includes a polyolefin microporous membrane. The water permeation efficiency of the polyolefin microporous membrane is 1.21 to 2.90 ml/min·cm², the bubble point of the polyolefin microporous membrane is 0.40 MPa to 0.65 MPa, and the compressibility of the polyolefin microporous membrane is less than 15%.

According to the invention as above, it is possible to provide a substrate for a liquid filter, which has excellent collection efficiency for particles of about 50 nm to 100 nm together with excellent liquid permeability, and also has stable liquid permeability for long-term use. Hereinafter, each element will be described in detail.

(Water Permeation Efficiency (Water Flow Rate))

The olefin microporous membrane, which is the substrate for a liquid filter of the invention, is characterized by its excellent flow rate characteristics. The water permeation efficiency of the olefin microporous membrane at a pressure differential of 90 kPa is 1.21 to 2.90 ml/min·cm². When the water permeation efficiency of the polyolefin microporous membrane is less than 1.21 ml/min·cm², sufficient water permeation efficiency as a liquid filter for particles of about 50 nm to 100 nm may not be obtained, resulting in problems, such as a decrease in the productivity of liquid filtration and an increase in the energy load required to maintain the flow volume (productivity). From such a point of view, the water permeation efficiency is more preferably 1.35 ml/min·cm² or more, and more preferably 1.45 ml/min·cm² or more. Meanwhile, when the water permeation efficiency of the polyolefin microporous membrane is more than 2.90 ml/min·cm², fine particles of about 50 nm to 100 nm may not be sufficiently collected, resulting in the problem that sufficient collection efficiency is not exerted. From such a point of view, the water permeation efficiency is more preferably 2.80 ml/min·cm² or less.

(Bubble Point)

The polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, is characterized in that fine particles of about 50 nm to 100 nm are collected at high level. The bubble point of the polyolefin microporous membrane is 0.40 MPa or more. In the invention, when the bubble point of the polyolefin microporous membrane is less than 0.40 MPa, fine particles as mentioned above cannot be sufficiently collected, and sufficient collection efficiency is not exerted. From such a point of view, the bubble point is more preferably 0.42 MPa or more. Meanwhile, when the bubble point of the polyolefin microporous membrane is more than 0.65 MPa, the resulting water permeation efficiency may be seriously insufficient, causing the problem that, for example, the flow through a filter using the polyolefin microporous membrane as a substrate is unstabilized, etc. From such a point of view, the bubble point is more preferably 0.63 MPa or less, and still more preferably 0.61 MPa.

(Compressibility)

The polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, is characterized by being capable of maintaining an excellent porous structure in the long-term use or processing of the liquid filter. It is necessary that the compressibility of the polyolefin microporous membrane is less than 15%, still more preferably 12% or less. Here, compressibility in the invention is determined as follows. A polyolefin microporous membrane is pressed at a temperature of 70° C. at 2 MPa for 30 seconds, and then the pressure is released at 25° C. for 30 seconds, followed by allowing to stand; from the difference in membrane thickness before and after pressing and also the ratio of the membrane thickness before pressing, the compressibility is determined. In the invention, when the compressibility of the polyolefin microporous membrane is 15% or more, in the long-term use or processing of the liquid filter, there may be problems in that the polyolefin microporous membrane is crushed more than necessary, whereby the original porous structure cannot be maintained, resulting in unstable water permeation efficiency, or that the filtration pressure abnormally increases due to pore blockage, for example. In addition, when the compressibility of the polyolefin microporous membrane is 2% or more, this results in the formation of a porous structure indispensable as a substrate for a liquid filter and thus is desirable.

Incidentally, in the invention, it is necessary that the above water permeation efficiency, bubble point, and compressibility are adjusted within appropriate ranges. Methods for controlling these properties are not particularly limit. An example thereof is the adjustment of production conditions, such as the average molecular weight of the polyolefin resin, the mixing ratio in the case where a plurality of polyolefin resins are mixed and used, the polyolefin resin concentration in the raw material, the mixing ratio in the case where a plurality of solvents are mixed and used in the raw material, the heating temperature and pressing pressure for solvent expression from the inside of an extruded sheet-like article, the draw ratio, the heat treatment (heat setting) temperature after drawing, and the immersion time in the extraction solvent, for example. In particular, as shown below in the description of the production method, they can be suitably obtained when the proportion of an ultrahigh-molecular-weight polyethylene in the entire polyethylene composition is 13 to 27 mass %, a suitable pressing pressure is applied with heating to 40 to 100° C. to express some of the solvent contained in an extruded sheet-like article, the total draw ratio is 40 to 120, or the heat set temperature is 132 to 143° C., for example.

(Heat Shrinkage)

It is preferable that the polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, has a shrinkage of 20% or more in the width direction (TD) after being allowed to stand at a temperature of 130° C. for 1 hour, still more preferably 20 to 35%, and particularly preferably 22 to 32%. In the case where the heat shrinkage of the polyolefin microporous membrane is 20% or more, during the processing of the polyolefin microporous membrane, when the membrane is conveyed in the situation of receiving a heat treatment, excellent conveying properties are likely to be obtained without slackening; therefore, this is desirable. Meanwhile, in the case where the heat shrinkage of the polyolefin microporous membrane is 35% or less, during the processing of the polyolefin microporous membrane, when the membrane is conveyed in the situation of receiving a heat treatment, excellent conveying properties are likely to be obtained without meandering or wrinkling; therefore, this is desirable.

(Thickness)

The polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, is thin but has excellent strength. It is preferable that the thickness thereof is 7 to 16 μm, still more preferably 8 to 15 μm. In the case where the thickness of the polyolefin microporous membrane is 7 μm or more, sufficient dynamic strength is likely to be obtained, making it easier to obtain handleability during the processing of the polyolefin microporous membrane, etc., or durability in the long-term use of a filter cartridge; therefore, this is desirable. Meanwhile, in the case where the thickness is 16 μm or less, sufficient water permeation efficiency is likely to be obtained with the microporous membrane alone. In addition, in a filter cartridge of a predetermined size, a larger filtration area can be obtained, making it easier to design the flow rate or structure of the filter during the processing of the polyolefin microporous membrane; therefore, this is desirable.

For example, assuming the placement of a filter cartridge in a housing of the same size, the thinner the filter medium (the entire components including the substrate for a filter) is, the larger the filter medium area can be, making it possible to achieve a high-flow-rate/low-filtration-pressure design, which is desirable as a liquid filter. That is, it becomes possible to design a liquid filter such that the filtration pressure decreases in the case where the same flow rate is to be maintained, while the flow rate increases in the case where the same filtration pressure is to be maintained. In particular, when the filtration pressure is low, it becomes much less likely that particles once collected are continuously exposed to the filtration pressure in the filter medium, thus are pushed out from the inside of the filter medium with the lapse of time together with the filtrate, and leak out. It also becomes much less likely that gas dissolved and present in the liquid to be filtered appears in the form of fine bubbles due to the pressure difference before and after filtration (pressure decrease after filtration). Further, this can also be expected to be effective in improving the filtration yield of the filtration object, such as a liquid chemical, and also maintaining its quality at high level over a long period of time.

Meanwhile, although the strength and durability of a filter medium decrease with a decrease in the thickness thereof, for example, if possible in designing a filter, it is also possible to combine the filter with a coarse high-strength support for reinforcement (e.g., processing to stack them and fold the stack) to adjust the designing of durability and flow rate.

(Pore-Blocking Temperature)

It is preferable that the polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, has a pore-blocking temperature of more than 140° C. In the case where the pore-blocking temperature of the polyolefin microporous membrane is more than 140° C., in the thermal bonding step during the processing of the polyolefin microporous membrane, near the high-temperature processing part or near the high-temperature-body contacting part, the porousness of the polyolefin microporous membrane is not lost, and the water permeation efficiency is maintained, whereby the desired filtration area is more likely to be obtained even after the processing; therefore, this is desirable.

(Porosity)

It is preferable that the polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, has a porosity of 50 to 60%, more preferably 52% to 58%. In the case where the porosity of the polyolefin microporous membrane is 50% or more, this results in excellent water permeation efficiency and thus is desirable. Meanwhile, in the case where the porosity is 60% or less, this provides the polyolefin microporous membrane with excellent dynamic strength and improved handleability, and thus is desirable. Here, the porosity (ε) of a polyolefin microporous membrane is calculated by the following equation. ε(%)={1−Ws/(ds·t)}×100

Ws: Weight per unit of polyolefin microporous membrane (g/m²)

ds: True density of polyolefin (g/cm³)

t: Thickness of polyolefin microporous membrane (μm)

(Polyolefin)

The polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, is a microporous membrane that contains a polyolefin. Here, “microporous membrane” means a membrane having a large number of micropores inside and configured such that the micropores are connected to each other, through which gas or liquid can pass from one side to the other side. It is preferable that the polyolefin microporous membrane has a polyolefin content of 90 mass % or more. As the remainder, organic or inorganic fillers, surfactants, and like additives may also be contained without affecting the advantageous effects of the invention.

Examples of polyolefins are homopolymers and copolymers of polyethylene, polypropylene, polybutylene, and polymethylpentene, mixtures of one or more kinds thereof, etc. Among them, polyethylene is preferable. As polyethylene, it is preferable to use a high-density polyethylene, a mixture of a high-density polyethylene and an ultrahigh-molecular-weight polyethylene, or the like. In addition, it is also possible to use polyethylene in combination with other components. Examples of components other than polyethylene include polypropylene, polybutylene, polymethylpentene, and copolymers of polypropylene and polyethylene. In addition, as a polyolefin, it is possible to use a combination of a plurality of polyolefins that are poorly compatible with each other and have different degrees of polymerization and branching, in other words, a plurality of polyolefins that are different in crystallinity, stretchability, and molecular orientation.

As a polyolefin for use in the invention, it is preferable to use a polyethylene composition containing 5 mass % or more of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 900,000 or more, still more preferably a composition containing 7 mass % or more of an ultrahigh-molecular-weight polyethylene, and particularly preferably a composition containing 13 to 27 mass % or more of an ultrahigh-molecular-weight polyethylene. In addition, when suitable amounts of two or more kinds of polyethylene are blended, upon drawing, this is effective in forming a network web structure associated with fibrillation and increasing the occurrence of pores. It is preferable that the mass average molecular weight after the blending of two or more kinds of polyethylene is 350,000 to 4,500,000, more preferably 2,500,000 to 4,500,000. In particular, a polyethylene composition obtained by mixing an ultrahigh-molecular weight polyethylene having amass average molecular weight of 900,000 or more as mentioned above with a high-density polyethylene having a mass average molecular weight of 200,000 to 800,000 and a density of 0.92 to 0.96 g/cm³ is preferable. In this case, it is preferable that the proportion of the high-density polyethylene in the polyethylene composition is 95 mass % or less, still more preferably 93 mass % or less, and particularly preferably 73 to 87 mass % (the ultrahigh-molecular-weight polyethylene is 13 to 27 mass %).

Incidentally, the mass average molecular weight can be determined as follows. A polyolefin microporous membrane sample is dissolved by heating in o-dichlorobenzene, and subjected to measurement by GPC (Alliance GPC 2000 manufactured by Waters Corporation, columns; GMH6-HT and GMH6-HTL) under the conditions of a column temperature of 135° C. and a flow velocity of 1.0 mL/min.

[Liquid Filter]

The substrate for a liquid filter of the invention mentioned above is, after suitable processing to impart compatibility with liquid chemicals, processed into a cartridge form, which can be used as a liquid filter. A liquid filter is an instrument for removing particles from a liquid to be treated containing particles made of organic substances and/or inorganic substances. In the liquid to be treated, particles are present in the form of a solid or a gel. The invention is suitable for the removal of particles having a particle size of about 50 nm to 100 nm. In addition, in addition to the semiconductor production step, the liquid filter may also be used in other production steps, such as display production and polishing, for example.

As a substrate for a liquid filter, a porous substrate made of polytetrafluoroethylene or polypropylene is well known, for example. The substrate of the invention mentioned above, which is composed of the polyolefin microporous membrane, has higher compatibility with liquid chemicals as compared with polytetrafluoroethylene porous substrates. Accordingly, the processing to impart compatibility with liquid chemicals to the filter is facilitated, for example. In addition, when the filter cartridge is placed in a filter housing, and the housing is filled with a liquid chemical, an air pocket is unlikely to be formed in the filter cartridge, resulting in improved yield of liquid chemical filtration. Further, because a polyethylene resin itself does not contain a halogen element, the used filter cartridge is easy to handle, which is effective in reducing the environmental impact, for example.

[Polyolefin Microporous Membrane Production Method]

The polyolefin microporous membrane, which is the substrate for a liquid filter of the invention, can be suitably produced by the method shown below. That is, it can be suitably produced by sequentially performing the following steps:

(I) a step of, in a solution containing a polyethylene composition and a solvent, preparing a solution containing a volatile solvent having a boiling point of less than 210° C. at least at atmospheric pressure;

(II) a step of melt-kneading the solution, extruding the obtained melt-kneaded article from a die, and cooling and solidifying the same to give a gel-like molded article;

(III) a step of, prior to drawing the gel-like molded article in at least one direction, expressing some of the solvent from the gel-like molded article;

(IV) a step of drawing the gel-like molded article in at least one direction; and

(V) a step of extracting and washing the solvent from the inside of the drawn intermediate molded article.

In the step (I), in which a solution containing a polyolefin composition and a solvent is prepared, a solution containing a volatile solvent having a boiling point of less than 210° C. at least at atmospheric pressure is prepared. Here, it is preferable that the solution is a thermally reversible sol-gel solution. That is, the polyolefin is dissolved by heating in the solvent to form a sol, thereby preparing a thermally reversible sol-gel solution. The volatile solvent having a boiling point of less than 210° C. at atmospheric pressure in the step (I) is not particularly limited as long as it is capable of sufficiently swelling or dissolving the polyolefin. Preferred examples thereof include liquid solvents, such as tetralin, ethylene glycol, decalin, toluene, xylene, diethyltriamine, ethylenediamine, dimethyl sulfoxide, and hexane. They may be used alone, and it is also possible to use a combination of two or more kinds. Among them, decalin and xylene are preferable.

In addition, in the preparation of the solution, in addition to the volatile solvent having a boiling point of less than 210° C. at atmospheric pressure mentioned above, it is also possible to add a non-volatile solvent having a boiling point of 210° C. or more, such as liquid paraffin, paraffin oil, mineral oil, or castor oil.

In the solution in the step (I), in terms of controlling the liquid permeation efficiency of the polyolefin microporous membrane and the removal efficiency as a filter medium, it is preferable that the concentration of the polyolefin composition is 10 to 25 mass %, still more preferably 12 to 23 mass %. In addition, with a decrease in the concentration of the polyolefin composition, the dynamic strength tends to decrease, resulting in poor handleability, and further, in the formation of a polyolefin microporous membrane, breakage tends to occur more frequently. In addition, with an increase in the concentration of the polyolefin composition, it tends to be more difficult to form pores.

In the step (II), the solution prepared in the step (I) is melt-kneaded, the obtained melt-kneaded article is extruded from a die, and then cooled and solidified to give a gel-like molded article. It is preferable that extrusion from a die is performed at a temperature within a range of the melting point of the polyolefin composition to the melting point +65° C. to give an extruded article, and then the extruded article is cooled to give a gel-like molded article.

It is preferable that the molded article is formed into a sheet shape. Cooling may be quenching with an aqueous solution or an organic solvent, or may also be casting on a cooled metallic roll. However, generally, quenching with water or the volatile solvent used for the sol-gel solution is employed. It is preferable that the cooling temperature is 10 to 40° C. Incidentally, it is preferable that a stream of water is established on the surface layer of a water bath, and a gel-like sheet is produced while preventing the mixed solvent released from a gelled sheet in the water bath and floating on the water surface from re-adhering to the sheet.

The step (III) is a step of, prior to drawing the gel-like molded article in at least one direction, expressing some of the solvent from the gel-like molded article. The step (III) can be suitably performed by, for example, applying pressure to the surface of the gel-like molded article by a method in which the article is passed through the gap between upper and lower two belts or rollers, etc. The amount of solvent to be expressed has to be adjusted according to the required liquid permeation efficiency and filtration object removal efficiency of the polyolefin microporous membrane. Such adjustment can be made within an appropriate range with the pressing pressure between the upper and lower belts or rollers, the temperature in the expression step, the number of presses, etc.

Incidentally, the pressure that the gel-like molded article receives is preferably adjusted to be 0.1 to 2.0 MPa in the case of using sheet-like objects such as belts. In the case of using rollers or the like, it is preferably performed at 2 to 45 kgf/m. It is preferable that the expression temperature is 40 to 100° C. In addition, the number of presses depends on the space allowance in the facility, and thus pressing can be performed without particular limitation.

In the step (III), as necessary, single-stage or multistage preheating may be performed before solvent expression to remove some of the volatile solvent from the inside of the sheet. In this case, it is preferable that the preheating temperature is 50 to 100° C.

The step (IV) is a step of drawing the gel-like molded article in at least one direction. Here, it is preferable that drawing in the step (IV) is biaxial drawing. Either of sequential biaxial drawing, in which longitudinal drawing and transverse drawing are separately performed, and simultaneous biaxial drawing, in which longitudinal drawing and transverse drawing are simultaneously performed, can be suitably employed. In addition, a method in which the article is drawn in the longitudinal direction several times and then drawn in the transverse direction, a method in which the article is drawn in the longitudinal direction and drawn in the transverse direction several times, and a method in which the article is sequentially biaxially drawn and then further drawn in the longitudinal direction and/or in the transverse direction once or several times are also preferable.

In terms of controlling the liquid permeation efficiency and filtration object removal efficiency of the polyolefin microporous membrane, it is preferable that the total draw ratio in the step (IV) (the product of the longitudinal draw ratio and the transverse draw ratio) is 40 to 120, more preferably 50 to 100. With an increase in the draw ratio, in the formation of a polyolefin microporous membrane, breakage tends to occur more frequently. In addition, with a decrease in the draw ratio, the thickness tends to be less uniform. As mentioned above, it is preferable that drawing is performed with the solvent remaining in a suitable state. It is preferable that the drawing temperature is 80 to 125° C.

In addition, after drawing in the step (IV), a heat set treatment may be performed. In terms of controlling the liquid permeation efficiency and filtration object removal efficiency of the polyolefin microporous membrane, it is preferable that the heat set temperature is 132 to 143° C. With an increase in the heat set temperature, the filtration object removal efficiency of the polyolefin microporous membrane tends to significantly decrease, while with a decrease in the heat set temperature, the liquid permeation efficiency tends to significantly decrease.

The step (V) is a step of extracting and washing the solvent from the inside of the drawn intermediate molded article. Here, in the step (V), in order to extract the solvent from the inside of the drawn intermediate molded article (drawn film), it is preferable that washing is performed with a halogenated hydrocarbon solvent such as methylene chloride or a hydrocarbon solvent such as hexane. In the case where washing is performed by immersion in a tank filled with a solvent, in order to obtain a polyolefin microporous membrane with low dissolution, it is preferable that washing is performed for 20 to 150 seconds, more preferably 30 to 150 seconds, and particularly preferably 30 to 120 seconds. Further, in order to further enhance the effect of washing, it is preferable that the tank is divided into several stages, and the washing solvent is poured from the downstream side of the step of conveying the polyolefin microporous membrane for the washing solvent to flow towards the upstream side of the conveying step so that the purity of the washing solvent in a downstream tank will be higher than in an upstream tank. In addition, depending on the performance requirements for the polyolefin microporous membrane, heat setting may be performed by an annealing treatment. Incidentally, in terms of conveying properties in the step, etc., it is preferable that the annealing treatment is performed at 50 to 150° C., still more preferably 50 to 140° C.

As a result of such a production method, a low-dissolution polyolefin microporous membrane having excellent liquid permeation efficiency and filtration object removal efficiency can be provided.

EXAMPLES

Hereinafter, examples of the invention, comparative examples, and various measurement methods will be described. However, the invention is not limited to these examples.

[Measurement Methods]

(Water Permeation Efficiency (Water Flow Rate))

A polyolefin microporous membrane was pre-immersed in ethanol and dried at room temperature. This polyolefin microporous membrane was set on a 37-mm-diameter liquid permeation cell made of stainless steel (liquid permeation area: S cm²). The polyolefin microporous membrane on the liquid permeation cell was wetted with a small amount (0.5 ml) of ethanol. Then, pre-weighed pure water V (100 ml) was passed therethrough at a pressure differential of 90 kPa, and the time T1 (min) taken for the entire amount of pure water to pass was measured. From the amount of pure water and the time taken for the passing of pure water, the water penetration volume Vs per unit time (min) and unit area (cm²) at a pressure differential of 90 kPa was calculated from the following equation and defined as water permeation efficiency (ml/min·cm²). The measurement was performed in a temperature atmosphere at room temperature 24° C. Vs=V/(T1×S) (Bubble Point)

The bubble point of a polyolefin microporous membrane was measured in accordance with ASTM E-128-61 using ethanol as the measurement solvent.

(Compressibility (Rate of Change of Membrane Thickness Before and after Pressing))

A polyolefin microporous membrane sample was cut to 47 mm×100 mm and pressed at a temperature condition of 70° C. at 2 MPa for 30 seconds. The thickness of the sample before pressing (pre-t) was measured, and the thickness of the sample allowed to stand at 25° C. for 30 minutes after pressing (post-t) was measured. From these thicknesses, the compressibility was calculated by the following equation. Incidentally, the thickness of the sample was measured using a contact thickness gage (manufactured by Mitutoyo Corporation, terminal diameter; 0.5 cm, terminal shape; cylindrical, measurement pressure; 0.1 N) at ambient temperature 24±2° C. Compressibility={(pre-t−post-t)/pre-t}×100(%) (Heat Shrinkage)

A polyolefin microporous membrane was cut to a size of 100 mm in width×100 mm in length such that each side is parallel to the length direction (MD) or the width direction (TD), and allowed to stand in an oven adjusted to a temperature of 130° C. for 1 hour. The resulting shrinkage in the width direction (TD) (dimensional change) was calculated by the following equation. Heat shrinkage (%)=(|dimension before heat treatment−dimension after heat treatment|/dimension before heat treatment)×100 (Thickness) A polyolefin microporous membrane was measured for thickness at 20 points using a contact thickness gage (manufactured by Mitutoyo Corporation), and the results were averaged to determine the thickness. Here, as a contact terminal, a terminal in a cylindrical shape with a 0.5-cm-diameter bottom was used. The measurement pressure was 0.1 N. (Pore-Blocking Temperature)

A cut polyolefin microporous membrane substrate was immersed in a methanol solution containing 3 mass % of a nonionic surfactant (manufactured by Kao Corporation; EMULGEN 210P) dissolved therein, followed by air-drying. The air-dried sample was sandwiched between SUS plates of a predetermined size, impregnated with 1 M LiBF₄ propylene carbonate/ethylene carbonate (mass ratio: 1/1), which is an electrolyte, and then enclosed in a 2032-type coin cell. Leads were connected to the coin cell, a thermocouple was attached thereto, and the coin cell was placed in an oven. While raising the temperature at a temperature rise rate of 1.6° C./min, the resistance of the cell was measured by an AC impedance method at an amplitude of 10 mV and a frequency of 100 kHz. The temperature at the time when the resistance reached 1,000 Ω·cm² was defined as the pore-blocking temperature.

(Porosity)

The porosity (ε) of a polyolefin microporous membrane was calculated by the following equation. ε(%)={1−Ws/(ds·t)}×100

Ws: Weight per unit of polyolefin microporous membrane (g/m²)

ds: True density of polyolefin (g/cm³)

t: Thickness of polyolefin microporous membrane (μm)

Incidentally, the weight per unit of a polyolefin microporous membrane was determined as follows. A sample of 10 cm×10 cm was cut out, the mass thereof was measured, and the mass was divided by the area to determine the weight per unit.

(Collection Efficiency)

100 ml of an aqueous solution containing 0.1 mass % of polystyrene particles (average particle size: 70 nm) was filtered through a polyolefin microporous membrane at a pressure differential of 10 kPa. From the difference between the mass of the 100-ml aqueous solution of polystyrene particles before filtration (M1) and the mass of the filtrate that had passed through the polyolefin microporous membrane (M2), the collection efficiency for polystyrene particles was determined. Incidentally, a collection efficiency of 90% or more was rated as the best (AA), 80% or more and less than 90% as good (A), and less than 80% as poor (B). Collection efficiency (%)=((M1−M2)/((M1×0.1×10⁻²))×100 (Rate of Change of Water Penetration Volume (Flow Stability))

A polyolefin microporous membrane was pre-immersed in ethanol and dried at room temperature. Five of the polyolefin microporous membranes were set on a 37-mm-diameter liquid permeation cell made of stainless steel (liquid permeation area: S cm²) on top of one another at 0.5-mm intervals. The polyolefin microporous membranes on the liquid permeation cell were wetted with a small amount (0.5 ml) of ethanol. Then, 200 ml of pure water was passed therethrough at a pressure differential of 40 kPa, and the time taken for the entire amount to pass (T1) was measured, immediately followed by the release of the pressure differential. Subsequently, using the same sample, the operation of passing 200 ml of pure water at a pressure differential of 40 kPa and then immediately releasing the pressure differential was repeated 100 times. The time taken by the 100^(th) passing of 200 ml of pure water (T100) was measured, and the rate of change of water penetration volume (%) was calculated from the following equation. Incidentally, a rate of change of water penetration volume of 10% or less was rated as the best (AA), more than 10% and 15% or less as good (A), and more than 15% as poor (B). Incidentally, it can also be understood that when the membrane is good in terms of the rate of change of water penetration volume, an excellent porous structure can be maintained for long-term use. Rate of change of water penetration volume (%)=(T100−T1)/T1×100 (Rupture Resistance)

A polyolefin microporous membrane was pre-immersed in ethanol and dried at room temperature. Three of the polyolefin microporous membranes were set on a 37-mm-diameter liquid permeation cell made of stainless steel (liquid permeation area: S cm²) on top of one another at 1.0-mm intervals, and 100 ml of ethanol was passed therethrough at a pressure differential of 30 kPa. After the entire amount had passed, the pressure differential was immediately released. Subsequently, using the same sample, the operation of passing 100 ml of ethanol at a pressure differential of 30 kPa and then immediately releasing the pressure differential was repeated 200 times. When no rupture occurred during the operation, the rupture resistance was rated as good (A), while when rupture occurred during the operation, the rupture resistance was rated as poor (B). It can be understood that when the membrane has good rupture resistance, an excellent porous structure can be maintained for long-term use.

(Dissolution Resistance)

A polyolefin microporous membrane was immersed in methylene chloride for a predetermined period of time, then the polyolefin microporous membrane was removed, and the mass of the methylene chloride solution after immersion was measured. Separately from this, new methylene chloride having the same mass as the measured mass was prepared. Methylene chloride was evaporated from each to completely remove the solvent (to dryness), and then the mass of each was measured. Based on the mass increment after the complete removal of new methylene chloride, the ratio of the mass increment after subjecting the methylene chloride solution, in which the polyolefin microporous membrane had been immersed, to evaporation to dryness was calculated. When the ratio was 1.05 or less, the dissolution resistance was rated as good (the filtrate was not contaminated, A), while when it was more than 1.05, the dissolution resistance was rated as poor (B).

Example 1

A polyethylene composition obtained by mixing 3 parts by mass of an ultrahigh-molecular-weight polyethylene having amass average molecular weight of 4,600,000 (PE1) and 12 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2) was used. A pre-prepared mixed solvent of 60 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 15 mass %, thereby preparing a polyethylene solution.

The polyethylene solution was extruded from a die into a sheet shape at a temperature of 160° C., and then the extruded article was cooled in a water bath at 25° C. At the same time, a stream of water was established on the surface layer of the water bath, and a gel-like sheet (base tape) was produced while preventing the mixed solvent released from a gelled sheet in the water bath and floating on the water surface from re-adhering to the sheet. The base tape was dried at 55° C. for 10 minutes and further at 95° C. for 10 minutes to remove decalin from the inside of the sheet. Subsequently, the sheet was conveyed on a roller heated to 85° C. while applying a pressing pressure of 20 kgf/m thereto, thereby removing some of liquid paraffin from the inside of the gel-like sheet. Subsequently, the base tape was drawn in the length direction at a temperature of 100° C. to 6 times the original length and successively drawn in the width direction at a temperature of 100° C. to 12 times the original length, immediately followed by a heat treatment (heat setting) at 136° C.

Next, the resulting drawn film was immersed in a two-tank methylene chloride bath successively for 30 seconds per tank, thereby extracting liquid paraffin. Incidentally, defining the tank where immersion is started as a first tank, and the tank where immersion is finished as a second tank, the purity of the washing solvent is (lower) first tank<second tank (higher). Subsequently, methylene chloride was removed by drying at 45° C., and the base tape was annealed while being conveyed on a roller heated to 110° C. to give a polyolefin microporous membrane.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 90% or more, and also had excellent water permeation efficiency and flow stability (the rate of change of water permeation volume: 10% or less).

The above production conditions are shown in Table 1, and the properties of the obtained polyolefin microporous membrane are shown in Table 2. Incidentally, the data of the following examples and comparative examples are also summarized in Tables 1 and 2.

Example 2

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except for the following. Using a polyethylene composition obtained by mixing 2 parts by mass of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 4,600,000 (PE1) and 18 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2), a pre-prepared mixed solvent of 55 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 20 mass %, thereby preparing a polyethylene solution.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 90% or more, and also had excellent water permeation efficiency and flow stability (the rate of change of water permeation volume: 10% or less).

Example 3

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the heat treatment (heat setting) temperature after biaxial drawing was 142° C.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 80% or more, and also had excellent water permeation efficiency and flow stability (the rate of change of water permeation volume: 10% or less).

Example 4

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except for the following. Using a polyethylene composition obtained by mixing 6.5 parts by mass of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 4,600,000 (PE1) and 18.5 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2), a pre-prepared mixed solvent of 50 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 25 mass %, thereby preparing a polyethylene solution. The polyethylene solution was extruded in the same manner as in Example 1, and the obtained gel-like sheet (base tape) was dried by heating and subsequently conveyed on a roller heated to 95° C. while applying a pressing pressure of 10 kgf/m thereto, thereby removing some of liquid paraffin from the inside of the gel-like sheet.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 90% or more, and also had excellent water permeation efficiency and flow stability (the rate of change of water permeation volume: 15% or less).

Comparative Example 1

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except for the following. Using a polyethylene composition obtained by mixing 0.5 parts by mass of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 4,600,000 (PE1) and 4.5 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2), a pre-prepared mixed solvent of 70 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 5 mass %, thereby preparing a polyethylene solution. This polyethylene solution was used, and the heat treatment (heat setting) temperature was 144° C.

The obtained polyolefin microporous membrane had a low bubble point, excessively high water permeation efficiency, and insufficient collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was less than 80%.

Comparative Example 2

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except for the following. Using a polyethylene composition obtained by mixing 9 parts by mass of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 4,600,000 (PE1) and 21 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2), a pre-prepared mixed solvent of 45 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 30 mass %, thereby preparing a polyethylene solution. This polyethylene solution was used, and the heat treatment (heat setting) temperature after biaxial drawing was 138° C.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 90% or more. However, the polyolefin microporous membrane had excessively low water permeation efficiency and insufficient flow stability (the rate of change of water permeation volume: more than 15%).

Comparative Example 3

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except for the following. Using a polyethylene composition obtained by mixing 12 parts by mass of a polyethylene copolymer having a mass average molecular weight of 150,000 and a propylene unit content of 1 mol % in place of the high-density polyethylene having a mass average molecular weight of 560,000 (PE2), a pre-prepared mixed solvent of 60 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 15 mass %, thereby preparing a polyolefin solution. The polyolefin solution was extruded, and the obtained base tape was dried at 55° C. for 10 minutes and further at 95° C. for 10 minutes to remove decalin from the inside of the base tape, and then conveyed on a heated roller without applying a pressing pressure. Subsequently, the base tape was drawn in the length direction at a temperature of 100° C. to 7 times the original length and successively drawn in the width direction at a temperature of 100° C. to 10 times the original length, immediately followed by a heat treatment (heat setting) at 130° C., and then immersed in a two-tank methylene chloride bath successively for 5 seconds per tank, thereby extracting liquid paraffin.

The obtained polyolefin microporous membrane had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 90% or more. However, the polyolefin microporous membrane had excessively low water permeation efficiency and insufficient flow stability (the rate of change of water permeation volume: more than 15%).

Comparative Example 4

A polyethylene composition obtained by mixing 4 parts by mass of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 4,600,000 (PE1) and 6 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2) was used. A pre-prepared mixed solvent of 55 parts by mass of liquid paraffin and 35 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 10 mass %, thereby preparing a polyethylene solution.

The polyethylene solution was extruded from a die into a sheet shape at a temperature of 160° C., and then the extruded article was cooled in a water bath at 25° C. At the same time, a stream of water was established on the surface layer of the water bath, and a gel-like sheet (base tape) was produced while preventing the mixed solvent released from a gelled sheet in the water bath and floating on the water surface from re-adhering to the sheet. The base tape was dried at 55° C. for 10 minutes and further at 95° C. for 10 minutes to remove decalin from the inside of the base tape. Subsequently, the base tape was conveyed on a roller heated to 85° C. while applying a pressing pressure of 20 kgf/m thereto, thereby removing some of liquid paraffin from the inside of the base tape. Subsequently, the base tape was drawn in the length direction at a temperature of 100° C. to 6 times the original length and successively drawn in the width direction at a temperature of 100° C. to 14 times the original length, immediately followed by a heat treatment (heat setting) at 135° C.

Next, the resulting drawn film was immersed in a two-tank methylene chloride bath successively for 30 seconds per tank, thereby extracting liquid paraffin. Incidentally, defining the tank where immersion is started as a first tank, and the tank where immersion is finished as a second tank, the purity of the washing solvent is (lower) first tank<second tank (higher). Subsequently, methylene chloride was removed by drying at 45° C., and the base tape was annealed while being conveyed on a roller heated to 110° C. to give a polyolefin microporous membrane.

The obtained polyolefin microporous membrane had excessively high compressibility. Although it had excellent collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was 80% or more, the flow stability and rupture resistance were insufficient, and thus the membrane was not suitable as a substrate for a liquid filter.

Comparative Example 5

A polyethylene composition obtained by mixing 2 parts by mass of an ultrahigh-molecular-weight polyethylene having amass average molecular weight of 4,600,000 (PE1) and 28 parts by mass of a high-density polyethylene having a mass average molecular weight of 560,000 (PE2) was used. A pre-prepared mixed solvent of 45 parts by mass of liquid paraffin and 25 parts by mass of decalin (decahydronaphthalene) was mixed with the polyethylene composition to a total polyethylene resin concentration of 30 mass %, thereby preparing a polyethylene solution.

The polyethylene solution was extruded from a die into a sheet shape at a temperature of 148° C., and then the extruded article was cooled in a water bath at 20° C. to give a gel-like sheet (base tape). The base tape was dried at 60° C. for 8 minutes and further at 95° C. for 15 minutes to remove decalin from the inside of the base tape. Subsequently, the base tape was drawn in the length direction at a temperature of 90° C. to 5.5 times the original length and successively drawn in the width direction at a temperature of 105° C. to 11 times the original length, immediately followed by a heat treatment (heat setting) at 130° C.

Next, the resulting drawn film was immersed in a methylene chloride bath for 10 seconds to extract liquid paraffin. Subsequently, methylene chloride was removed by drying at 50° C., and the base tape was annealed while being conveyed on a roller heated to 120° C. to give a polyolefin microporous membrane.

The obtained polyolefin microporous membrane had excessively high compressibility, insufficient collection efficiency such that the rate of collection of polystyrene particles having a particle size of 70 nm was less than 80%, and insufficient flow stability, and thus was not suitable as a substrate for a liquid filter.

Comparative Example 6

A T-die sheet was prepared from a mixture of a high-molecular-weight polyethylene having a mass average molecular weight of 5,000,000 (intrinsic viscosity [η]=16.5 dl/g) and paraffin wax (melting point: 69° C.). Here, the raw materials were mixed in the following ratio: high-molecular-weight polyethylene:paraffin wax=3:7. Next, paraffin wax was extracted for 10 minutes in n-decane at 80° C., followed by drying in an air oven at 60° C. The extraction and drying were performed with the direction of the length of the sheet being fixed. Subsequently, biaxial drawing was performed to give a porous film.

The obtained polyolefin microporous membrane had excessively high water permeation efficiency, excessively high compressibility, and insufficient flow stability, and thus was not suitable as a substrate for a liquid filter.

Comparative Example 7

An inflation film having a thickness of 45 μm (draft ratio: 8.7, inflation ratio: 8) was prepared from a high-molecular-weight polyethylene having a mass average molecular weight of 5,000,000 (intrinsic viscosity [η]=16.5 dl/g). Next, with the four directions of the film being fixed, the film was treated in n-paraffin (melting point: 28.1° C.) at 128° C. for 2 minutes and then immersed in n-hexane for 2 hours to completely extract n-paraffin, and then n-hexane was dried in an air oven, followed by fixed-width uniaxial drawing at a drawing speed of 1.5 m/min in air, thereby giving a porous film.

The obtained polyolefin microporous membrane had a low bubble point, excessively high water permeation efficiency, insufficient collection efficiency for polystyrene particles having a particle size of 70 nm, and insufficient flow stability, and thus was not suitable as a substrate for a liquid filter.

Comparative Example 8

A dry blend of 20 parts by mass of a high-molecular-weight polyethylene having a mass average molecular weight of 4,400,000 (viscosity average molecular weight: 2,000,000) and 80 parts by mass of granular stearyl alcohol was fed to an extruder, and, with kneading at 240° C., continuously extruded from a T-die having a width of 550 mm and a die clearance of 0.4 mm to give a sheet having a thickness of 150 μm. The sheet was subjected to stearyl alcohol extraction in an isopropyl alcohol bath at 60° C. to give a porous membrane made of polyethylene having a thickness of 120 μm. The membrane was drawn in the length direction at 100° C. to 3.5 times the original length and then drawn in the direction perpendicular to the length direction at 125° C. to 9.1 times the original length, thereby giving a porous membrane having a thickness of 10 μm.

The obtained polyolefin microporous membrane had a low bubble point, excessively high water permeation efficiency, insufficient collection efficiency for polystyrene particles having a particle size of 70 nm, and insufficient flow stability, and thus was not suitable as a substrate for a liquid filter.

TABLE 1 Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative Example 1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 3 Example 4 Example 5 Compo- Decalin (part by mass) 25 25 25 25 25 25 25 35 25 sition Paraffin (part by mass) 60 55 60 50 70 45 60 55 45 of So- PE concentration 15 20 15 25 5 30 15 10 30 lution (mass %) PE 1 (part by mass) 3 2 3 6.5 0.5 9 3 4 2 PE 1 Mw 4,600,000 4,600,000 4,600,000 4,600,000 4,600,000 4,600,000 4,600,000 4,600,000 4,600,000 PE 2 (part by mass) 12 18 12 18.5 4.5 21 12 6 28 PE 2 Mw 560,000 560,000 560,000 560,000 560,000 560,000 150,000 560,000 560,000 Extru- Die temperature (° C.) 160 160 160 160 160 160 160 160 148 sion Cooling temperature 25 25 25 25 25 25 25 25 20 (° C.) Ex- First drying 55 55 55 55 55 55 55 55 60 pres- temperature sion (° C.) First drying time 10 10 10 10 10 10 10 10 8 (min) Second drying 95 95 95 95 95 95 95 95 95 temperature (° C.) Second drying time 10 10 10 10 10 10 10 10 15 (min) Expression 85 85 85 95 85 85 — 85 — temperature (° C.) Expression pressure 20 20 20 10 20 20 — 20 — (kgf/m) Drawing Longitudinal drawing 100 100 100 100 100 100 100 100 90 temperature (° C.) Longitudinal draw 6 6 6 6 6 6 7 6 5.5 ratio Transverse drawing 100 100 100 100 100 100 100 100 105 temperature (° C.) Transverse draw ratio 12 12 12 12 12 12 10 14 11 Heat set temperature 136 136 142 136 144 138 130 135 130 (° C.) Extraction Extraction time (sec) 60 60 60 60 60 60 10 60 10 Drying temperature 45 45 45 45 45 45 45 45 50 (° C.) Annealing 110 110 110 110 110 110 110 110 120 temperature (° C.)

TABLE 2 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 Thickness (μm) 11 11 14 9 14 10 12 15 13 30 96 10 Weight per 4.7 5.0 5.4 4.2 5.3 5.1 6.0 5.3 6.0 10.1 21.2 2.9 Unit Area g/m² Porosity (%) 55 53 60 51 61 47 48 63 52 65 77 70 Bubble Point 0.50 0.55 0.42 0.62 0.30 0.67 0.72 0.37 0.3 0.18 0.07 0.41 (MPa) Water 2.20 1.70 2.85 1.40 3.20 1.22 0.65 1.90 0.50 1.71 0.81 2.66 Permeation Efficiency (ml/min · cm²) Compress- 7 9 12 4 18 3 13 16 18 25 35 30 ibility (%) 130° C. TD 25 28 21 32 15 30 37 33 30 35 40 30 Heat Shrinkage (%) Pore-Blocking 143 142 141 146 140 145 144 140 140 145 148 140 Temperature (° C.) 70-nm AA AA A AA B AA AA A B B B A Collection Efficiency Flow Stability AA AA AA A AA B B A B B B B Rupture A A A A A A A B A B B B Resistance Dissolution A A A A A A B A B B B B Resistance 

The invention claimed is:
 1. A substrate for a liquid filter, comprising a polyolefin microporous membrane, the polyolefin microporous membrane having a water permeation efficiency of 1.21 to 2.90 ml/min·cm² at a pressure differential of 90 kPa, the polyolefin microporous membrane having a bubble point of 0.40 MPa to 0.65 MPa, the polyolefin microporous membrane having a compressibility of less than 15%, the polyolefin microporous membrane having a thickness of 7 to 16 μm, the polyolefin being a composition consisting of 10 to 26 mass % of an ultrahigh-molecular-weight polyethylene having a mass average molecular weight of 900,000 or more and 74 to 90 mass % of a high-density polyethylene having a mass average molecular weight of 200,000 to 800,000, and the polyolefin microporous membrane having a porosity of 50 to 60%.
 2. The substrate for a liquid filter according to claim 1, wherein the polyolefin microporous membrane has a heat shrinkage of 20% or more in the width direction after a heat treatment at 130° C. for 1 hour.
 3. The substrate for a liquid filter according to claim 2, wherein the polyolefin microporous membrane has a pore-blocking temperature of more than 140° C.
 4. The substrate for a liquid filter according to claim 1, wherein the polyolefin microporous membrane has a pore-blocking temperature of more than 140° C.
 5. The substrate for a liquid filter according to claim 1, wherein the substrate is obtained by preparing a solution containing a polyethylene composition and a solvent including a volatile solvent having a boiling point of less than 210° C. at least at atmospheric pressure; melt-kneading the solution, extruding the obtained melt-kneaded article from a die, and cooling and solidifying the same to give a gel-like molded article; prior to drawing the gel-like molded article in at least one direction, expressing some of the solvent from the gel-like molded article; drawing the gel-like molded article in at least one direction; heat set treating the drawn gel-like molded article at heat set temperature of from 132° C. to 143° C. to form a drawn intermediate molded article; and extracting and washing the solvent from inside of the drawn intermediate molded article. 